The invention disclosed relates to a method of pasteurizing and sterilizing foodstuffs utilizing microwaves, and to an oven for implementing such a process, that is, a new microwave process by which to reduce or eliminate bacteria from foods for human and animal consumption, and thus prolong the period for which they can be preserved; both the method and the oven used for its implementation constitute the subject matter of the application.
In addition to the long-standing hot air, hot water and vapor methods of pasteurization (85.degree. C.) and sterilization (121.degree. C.), and to methods of deep freezing (-25.degree. C.), the prior art also embraces a type of treatment utilizing microwaves generated at the conventional frequency of 2.450 MHz.
Traditional hot air, hot water and vapor methods are beset by several drawbacks, namely long exposure times, lack of continuity, a cooked effect produced by the high temperatures adopted to speed up the process, non-uniform temperatures, deterioration of or damage to wrappings, and unsatisfactory results in the case of solid foods, due to the slow transfer of heat by conduction.
There are also drawbacks with the deep-freezing of foodstuffs, most notably the greater complexity and cost of equipment as compared with that utilized for pasteurization and sterilization.
Microwave treatment of foodstuffs involves the application of heat by inducing molecular vibration throughout the entire mass of the edible substance, and thus brings a number of theoretical advantages: pasteurization or sterilization can be achieved in a much shorter time than is possible by applying heat directly; the organoleptic properties of the food are improved, as also is its appearance and that of its wrapping; labor costs are cut by virtue of the automation obtainable; the cooked effect is reduced; sterilization of solids can be achieved without the need to use regulating fluids, and pasteurization is made possible without excessive damage occurring to wrappings as a result of internal pressure generated by evaporation and heat. The only disadvantage is that the wrappings utilized must be transparent, wholly or in part, to microwave radiation.
Notwithstanding these apparent advantages, the attempts made thus far at microwave pasteurization and sterilization on an industrial scale have not been such as to develop a continuous pasteurization method whereby packaged foodstuffs can be subjected to uniform and stable heat throughout the entire area of the oven; neither has sterilization been successful, inasmuch as the requisite temperature level tends to prduce internal gas pressures of an order that often causes the wrapping or container to explode.
Accordingly, microwave methods do not constitute a viable proposition at industrial level.
The prior art as summarized above thus stands in need of considerable improvement, the objects being to eliminate the aforementioned drawbacks besetting microwave pasteurization, and to enable the use of microwave radiation in effecting sterilization on industrial scale.
It will be discerned from the foregoing that the requirement exists for a new method of microwave treatment capable of being operated continuously, of ensuring that each container or wrapper containing the foodstuff will be invested with a constant, uniform heat, according to the temperature selected, without bursting or becoming distorted (even at high sterilization temperatures), and of turning out an end-product substantially fresher than is obtainable conventionally, in less time and with less waste.